Reference positions within an exposure apparatus for producing relatively large patterns, such as are required to produce a large-scale liquid crystal display panel, may be determined by means of one or more reference marks arranged on a plate stage. Using such a reference mark on the plate stage, a reticle positioning mark (or marks) on a reticle are compared to the reference mark through an optical projection system, and the position of the reticle, as projected through the optical projection system, is thus measured relative to a plate-stage coordinate system.
In an exposure apparatus for exposing patterns, such as for a liquid crystal panel, in which a single large image must be formed using a plurality of reticles, each of the plurality of reticles is loaded successively onto a reticle stage, positioned by a reticle alignment system, and measured relative to a plate-stage coordinate system. In this way, a baseline is obtained for each reticle before any exposure is made.
Conventional methods to detect the position of each reticle include a method as described in Japanese Patent Publication No. 61-143760. According to this method, the relative position between the reticle and the plate stage is detected by the use of reticle positioning marks on the reticle and reference marks on the plate stage, with a light-receiving sensor disposed under the reference marks. Another conventional method is described in Japanese Patent Publication No. 63-284814 and U.S. Pat. No. 4,943,73, wherein a slit on the reference mark on the plate stage receives light from the plate stage side, and the relative position of a slit-type reticle positioning mark is detected through an optical projection system by a light-receiving sensor located inside an illuminating optical system.
The reference marks on the plate stage are conventionally placed at the side of the top surface of the stage, on a raised platform or rim next to the location at which a substrate is to be supported, such that the marks are approximately at the height of the image plane. In an exposure apparatus for use with an especially large, generally square-shaped substrate, such conventional reference marks have the disadvantages of increasing the width and weight of the stage, and of requiring a large range of stage movement to bring the marks to various measurement locations.
An exposure apparatus of the type for exposing large substrates also typically includes a plate alignment system for aligning a plate. (A plate is a substrate, e.g., glass, on which a display is to be formed, typically having been prepared with a photosensitive coating thereon). When a device layer is to be exposed on a plate, the plate alignment system detects the position of the plate, or the position on the plate of a previously exposed device layer. In preparation for such use of the plate alignment system, the one or more reference marks are used to measure the position of the plate alignment system. The baseline of the plate alignment system in an exposure apparatus is obtained in this way.
Normally, before an exposure is made, after executing the above baseline measurement processes for the plate alignment system, and for the reticles to be used, the plate alignment system monitors plate alignment marks on a plate that is transported onto the plate stage, and thereby measures the plate position. The exposure apparatus then properly positions the plate, using the plate alignment system, and the reticle(s), using the reticle alignment system, according to the measured baseline values, and then performs an exposure.
Also during exposure, twisting (yawing) of the plate stage, which can occur when the stage steps from one substrate exposure location to another, may be corrected. The twisting is measured by the interferometer system of the plate stage. The reticle is rotated appropriately to compensate for the measured twisting by determining an offset of a reticle alignment mark relative to an index mark inside the reticle alignment system, and rotating the reticle to this offset position by, for example, a motor or a push-pull spring.
According to the above-described technology, when using a plurality of reticles, time is required to exchange the reticle and to detect the position of the plate alignment system and reticle. Additional time is required to inspect for foreign matter on the reticle. Although a total of four reticles has been used for one pattern layer, because of device optimization and larger panel sizes in recent years, it is becoming necessary to provide six or more reticles for a single pattern layer. In an apparatus utilizing a reference mark to detect the relative position of a reticle loaded on a reticle stage and the position of a plate alignment system, use of additional reticles increases the time required to measure the relative position of each reticle and the position of the plate alignment system. During this time, forces due to movement of the stage and thermal variations result in drifting of the position of the reference mark itself. This leads to a problem wherein the measured values change increasingly with passage of time.
Certain problems have also existed in standard reticle alignment systems. Because a standard reticle alignment system itself is formed on quite a large base and the relative positions of the index mark and the reticle alignment mark are measured through an optical system, relative deformation of the base of the reticle alignment system may be caused by temperature variations over time. Changes to the optical system of the reticle alignment system may also occur over time, as well as fluctuations due a relatively long space between the index mark located inside the reticle alignment system and the reticle alignment mark of the reticle. Vibrations of the apparatus can also cause fluctuations and drift in the reticle alignment system.